The present invention relates to a method of knock recognition. A method of this type is already known. In this known method of knock recognition, a common knock sensor on the engine block is associated with a four-cylinder engine of an internal combustion engine. This knock sensor records, for each cylinder, the noise level in the cylinder concerned over a certain crankshaft angle. The knock sensor signal recorded in this way is integrated over this crankshaft range. The signals which the knock sensor supplies are amplified for the individual cylinder. Because the different cylinders are heard with different loudness levels by the knock sensor, because of its installation location, different reference levels result for the cylinders. The amplification of the recorded reference level takes place with different factors on the basis of the different sound intensities. For an objective comparison of the individual reference levels RP, a standardized reference level RPn is determined in which the reference level RP is divided by the amplification factor of the individual cylinder. These standardized reference levels in this way form the basis for the knock recognition because measured knock integrals are analyzed which are referred to the reference level in normal operation, i.e. when the engine is running without knocking. In this known method, the average RPn of all the standardized reference levels RPn is first formed. The quotient of the average RPn of all the cylinders and of the standardized reference levels RPn of a cylinder determines the region in a knock recognition factor characteristic diagram from which a knock recognition factor (K factor) is to be taken. This K factor characteristic diagram is arranged in terms of the standardized reference level RPn and the rotational speed. The K factor (K) determined is multiplied by the reference level RP and, by this means, determines the knock threshold.
In this method, the selection of the K factor is erroneous because the sensor hears noises in a certain cylinder, which are actually normal, as being louder because of the installation location in the engine block. This means that the sensor emits a higher reference level for analysis in the case of a cylinder which can be heard easily. In consequence, the quotient of the standardized reference level of this cylinder and the average RPn is greater than that for the other cylinders. Because of this, the K factor is taken from a different region of the characteristic diagram so that the knock threshold for this cylinder is reduced. As a result, even a normal noise level can erroneously lead to knock recognition. In the reverse case, the knock threshold would be raised for a cylinder which is less easily heard because of an unfavourable sensor installation location and, in consequence, supplies smaller knock integrals. Actual knocking in this cylinder would not, under certain circumstances, be recognized.
In addition, a method of knock recognition is known from EP 0 098 584 in which the knock signals filtered by means of a band pass are compared with a reference value by a comparator. If the knock signals are larger than the reference value, a pulse is emitted by the comparator. These pulses are integrated and compared with a knock threshold fixed for the specific cylinder. This knock threshold fixed for the specific cylinder is a direct decision criterion. This method does not permit the sensor sensitivity to be taken into account because the parameters individual to the cylinder are not already taken into account before the comparison with the reference level.